कल्पान्तरे विश्वासटिप्पन्यः

Sources of the dharma include the veda-s, itihAsa-s, purANa-s, the vedAnga called kalpa (which includes dharmasUtra-s, smRtis, dharmashAstra-s and their commentaries).
General intro to valid sources of hindu knowledge here.
These sources are classifified into shruti (veda-s), smRti texts and shiShTAchAra (practice of learned elders).
- All of these are ultimately said to derive from the veda-s in some traditions - hypothesizing lost veda-s and brAhmaNa-s in the process.
Among the smRti-s
- manu-smRti and yAJNavalkya-smRti were the most important classically (going by the number of commentaries).
- parAshara-smRti is considered by itself (the beginning verses) and many (eg. vidyAraNya and P V Kane) as the most suitable for the kali age, due to its flexibility. Especially because it lacks a vyavahAra section. [UPSC05 IM] From its recommendations regarding kShatriya-s, for example, one can allow for the existence of kShatriya-s even in the kali age.
From a shaaastric view, all shaakhaas and all smRtis are authoritative to all (Mimaamsaa: SaakhaantaraadhikaraNam) with some rules for conflict resolution.
- Primacy of veda-s relative to other shAstra-s - see holy books page here.
Sectarian dharmottara-s aim to faithfully summarize the dharma prescribed in the other sources.
- shivadharmottara - “पुराणं भारतं वेदाः शास्त्राणि सुमहान्ति च/ आयुषः क्षपनायैव धर्मो ऽल्पग्रन्थसंस्थितः॥”
Random anubhava is not traditionally considered a source of dharma. It has to be established by tarka based on texts (led by veda-s) and shiShTAchAra practices of elders.
- For example: “न धर्मजिज्ञासायामिव श्रुत्यादय एव प्रमाणं ब्रह्मजिज्ञासायाम् । किन्तु श्रुत्यादयोऽनुभवादयश्च यथासम्भवमिह प्रमाणम्, अनुभवावसानत्वाद्भूतवस्तुविषयत्वाच्च ब्रह्मज्ञानस्य ।” (shankara in brahmasUtrabhAShyam 1.1.2).
- abhinavagupta on BG 2.46 described in holy books page here.

Completeness

To (surprisingly) great extant,
shAstra-s do actually cover problems even in modern contexts.
It is just that modern people are severely illiterate in dharmashAstras.

मातापितृसहस्रेभ्योऽपि वत्सलतरं शास्त्रम्

इति रामानुजः।
shAstra is very considerate (eg. 7 types of snAna ranging from river bAth to mental bath).

However much one who has deviated from it,
one can begin one’s return;
and one observes near-immediate benefits due to it

Extensions

For what’s not explicitly covered in the shAstra-s,
there are general principles which can be invaluable (they tend to be philosophical).